The present invention relates to improved lead-free electrodeposition baths containing a resinous phase dispersed in an aqueous medium, the resinous phase comprised of an ionic electrodepositable resin, a curing agent therefor, and a calcium compound present in a specified amount; and to their use in the method of electrodeposition. More particularly, this invention relates to electrodeposition baths which provide improved corrosion resistance, especially for untreated steel substrates.
Electrodeposition as a coating application method involves deposition of a film-forming composition onto a conductive substrate under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because, by comparison with non-electrophoretic coating means, electrodeposition offers increased paint utilization, improved corrosion protection and low environmental contamination.
Initially, electrodeposition was conducted with the workpiece being coated serving as the anode. This was familiarly referred to as anionic electrodeposition. However, in 1972, cationic electrodeposition was introduced commercially. Since that time, cationic electrodeposition has steadily gained in popularity and today is by far the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition.
Typically, electrodepositable coatings comprise an electrodepositable film-forming polymer and a curing agent, in combination with, inter alia, pigments. Lead-containing pigments such as lead silica chromate, basic lead silicate, lead chromate, and lead sulfate are often used in electrodepositable coatings because they impart excellent corrosion resistance to the electrocoated article. However, the acid used in cationic electrodeposition baths often solubilizes a portion of the lead pigment forming lead salts which are soluble in the aqueous phase of the electrodeposition bath. These lead salts often find their way into the ultrafiltrate of the bath, thus necessitating the removal and subsequent disposal of metallic lead and/or ionic or organic lead-containing materials.
In recent years, due to environmental concerns, particularly in Europe and Japan, the use of lead-free coatings has been mandated. Although surface coatings of excellent quality can be achieved by means of cationic electrodeposition of lead-free coatings, the removal of corrosion inhibitive lead pigments can result in poor corrosion resistance of these coatings, particularly when applied to untreated steel substrates.
The use of calcium-containing pigments to improve corrosion resistance of coatings is well known in the art. However, the effectiveness of calcium in the cationic electrodeposition of lead-free coatings is not known. It, therefore, would be advantageous to provide a lead-free electrodeposition bath which provides improved corrosion resistance of the electrocoated metal substrates, especially untreated steel.